Glossary beginning with H

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h

Hardness

The resistance of a mineral to scratching.

Harmonic Tremor

A continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault.

Heat transfer

Movement of heat from one place to another.

Heterolithologic

Material is made up of a heterogeneous mix of different rock types. Instead of being composed on one rock type, it is composed of fragments of many different rocks.

Holocene

The time period from 10,000 years ago to the present. Also, the rocks and deposits of that age.

Horizontal Blast

An explosive eruption in which the resultant cloud of hot ash and other material moves laterally rather than upward.

Horst

A block of the earth's crust, generally long compared to its width, that has been uplifted along faults relative to the rocks on either side.

Hot Spot

A volcanic center, 60 to 120 miles (100 to 200 km) across and persistent for at least a few tens of million of years, that is thought to be the surface expression of a persistent rising plume of hot mantle material. Hot spots are not linked to arcs and may not be associated with ocean ridges.

Hot-spot Volcanoes

Volcanoes related to a persistent heat source in the mantle.

Hyaloclastite

A deposit formed by the flowing or intrusion of lava or magma into water, ice, or water-saturated sediment and its consequent granulation or shattering into small angular fragments.